Any antenna to be implemented with mobile terminals (e.g., mobile telephones) or on PCs should be small. The antenna should also support a broadband in order to cope with a plurality of wireless communication systems.
The antenna apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 2005-203878 has a multi-resonant antenna that supports a plurality of wireless communication systems. More specifically, this antenna apparatus has an additional antenna element that shares a feeding point, with a folded monopole antenna, thus forming a multi-resonant antenna.
In the antenna apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 2006-279530, an impedance adjusting module adjusts the operating frequency of the antenna apparatus. More precisely, the impedance at the open end of the antenna element is adjusted, thereby adjusting the operating frequency, in this antenna apparatus. The antenna apparatus supports a broadband because of the adjustment of operating frequency.
If the antenna apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 2005-203878 is made smaller, the band each element of the multi-resonant antenna supports will decrease, ultimately making it difficult to ensure a part of the band. With regard to the frequency-tunable antenna (i.e., reconfigurable antenna), a configuration is known, which uses an active element, such as a switch, to switch the element connected to the antenna apparatus, from one to another, as in the antenna apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 2006-279530, thereby adjusting the operating frequency of the antenna. However, the loss at the active element adversely influences the antenna radiative efficiency. This influence is more prominent as the antenna apparatus is miniaturized, reducing the radiative resistance of the antenna elements. Further, if the antenna apparatus is implemented by a multi-resonant antenna and if the frequency-tunable technique is applied to the antenna apparatus, resonant frequency that needs to be adjusted may coexist with resonant frequency that need not be adjusted. Moreover, from the viewpoint of implementation, it is desired that any antenna apparatus to be implemented with a mobile terminal or a PC should have only one feeding point, i.e., common feeding point.